This application addresses broad Challenge Area (11) Regenerative Medicine and specific Challenge Topic, 11-DK-105: Transdifferentiation or directed reprogramming of one cell type to another. In work to understand how blood is patterned in early mesoderm, we have discovered that a cocktail of transcription factors interacts synergistically to cause cells that do not have the potential to become blood to convert to blood at high efficiency. When coexpressed during monolayer differentiation of ES cells, conditions under which no blood is ever made, this cocktail drives approximately 85% of total cells into the hematopoietic lineage within 4 days of induction. This efficiency of differentiation is unprecedented, and several lines of evidence indicate that it is due to reprogramming the fates of cells that were specified into other lineages. We propose to explore this lineage-specific reprogramming mechanistically with the objective of developing the technology to reprogram differentiated (or undifferentiated) cells of other lineages into repopulating hematopoietic stem cells useful for therapy. PUBLIC HEALTH RELEVANCE: We propose exploratory experiments to develop a method of reprogramming cells of other lineages to the blood lineage, with the objective of finding alternate sources of hematopoietic stem cells for bone marrow transplantation. If successful, direct reprogramming to blood-forming stem cells would enable immune tolerance for organ transplantation, genetic correction and syngeneic transplantation for genetic diseases of the blood, and expand the donor pool for patients in need of bone marrow transplantation.